The third generation partnership project (3GPP) has lately initiated the Long Term Evolution (LTE) program to bring new technology, new network architecture, new configuration and new applications and services to the wireless cellular network in order to provide improved spectral efficiency and faster user experiences.
Universal Mobile Telecommunications System (UMTS) Ciphering Architecture
In UMTS systems such as Release 6, the ciphering function is performed either in the Radio Link Controller (RLC) sub-layer or in the Medium Access Control (MAC) sub-layer, according to the following rules:                If a radio bearer is using a non-transparent RLC mode (Acknowledged Mode (AM) or Unacknowledged Mode (UM)), ciphering is performed in the RLC sub-layer.        If a radio bearer is using the transparent RLC mode, ciphering is performed in the MAC sub-layer (dedicated MAC (MAC-d) entity).        
For uplink traffic, ciphering is done in the Wireless Transmit/Receive Unit (WTRU) and deciphering in the Radio Network Controller (RNC), and for downlink traffic, ciphering is done in the RNC and deciphering in the WTRU.
The ciphering and integrity protection algorithms are described in 3GPP TS 33.102 V6.5.0 (2005-12). Of particular relevance are the COUNT-C (ciphering) and COUNT-I (integrity) input parameters, which are used by the ciphering and integrity protection algorithms respectively.
As shown in FIG. 1, a COUNT-C configuration, where the COUNT-C is utilized for performing ciphering on radio bearers using RLC TM 10 and COUNT-C is utilized for performing ciphering on radio bearer using RLC AM 30, or a RLC UM 20. For all transparent mode RLC radio bearers of the same core network (CN) domain, COUNT-C is the same, and COUNT-C is also the same for uplink and downlink.
The ciphering sequence number COUNT-C is 32 bits long. COUNT-C is composed of two parts: a “short” sequence number and a “long” sequence number. The “short” sequence number forms the least significant bits of COUNT-C while the “long” sequence number forms the most significant bits of COUNT-C, and is known as the Hyper Frame Number (HFN).
The update of COUNT-C depends on the transmission mode as described below:                For RLC TM on Dedicated Channel (DCH), the “short” sequence number is the 8-bit connection frame number (CFN) of COUNT C. It is independently maintained in the WTRU MAC-d entity and the Servicing RNC (SRNC) MAC-d entity. The “long” sequence number is the 24-bit MAC-d HFN, which is incremented at each CFN cycle.        For RLC UM mode, the “short” sequence number is the 7-bit RLC sequence number (RLC SN) and this is part of the RLC UM Protocol Data Unit (PDU) header. The “long” sequence number is the 25-bit RLC UM HFN which is incremented at each RLC SN cycle.        For RLC AM mode, the “short” sequence number is the 12-bit RLC sequence number (RLC SN) and this is part of the RLC AM PDU header. The “long” sequence number is the 20-bit RLC AM HFN which is incremented at each RLC SN cycle.        
The hyperframe number HFN is initialized by means of the parameter START. The WTRU and the RNC then initialize the 20 most significant bits of the RLC AM HFN, RLC UM HFN and MAC-d HFN to START. The remaining bits of the RLC AM HFN, RLC UM HFN and MAC-d HFN are initialized to zero. The hyper frame number is not explicitly transmitted with the packet.
Similarly, COUNT-I is composed of two parts: a “short” sequence number and a “long” sequence number. The “short” sequence number forms the least significant bits of COUNT-I, while the “long” sequence number forms the most significant bits of COUNT-I. The “short” sequence number is the 4-bit Radio Resource Control (RRC) sequence number (RRC SN) that is available in each RRC PDU. The “long” sequence number is the 28-bit RRC hyper frame number (RRC HFN) which is incremented at each RRC SN cycle.
LTE Architecture
FIG. 2 shows known LTE Layer 2 architecture in which the ciphering is done in the Packet Data Convergence Protocol (PDCP) layer 210 according to the current LTE architecture. The PDCP layer is located in the WTRU for the uplink traffic case, and used to be located in the access gateway (aGW) for the downlink traffic case according to the prior LTE decision.
Recently the Radio Access Network (RAN) WG3 in 3GPP made a decision to move ciphering and PDCP functionalities from the access gateway (aGW) to the enhanced Node B (eNB). As a result of this decision several issues need to be addressed in the area of devising a ciphering architecture that is effective and efficient.
RLC
Some of the main services and functions of the RLC sub-layer include:                Transfer of upper layer PDUs supporting AM or UM;        TM data transfer;        Error Correction through automatic repeat request (ARQ) (Cyclic Redundancy Check (CRC) check provided by the physical layer, in other words no CRC needed at RLC level);        Segmentation according to the size of the Transport Block (TB): only if an RLC Service Data Unit (SDU) does not fit entirely into the TB then the RLC SDU is segmented into variable sized RLC PDUs, which do not include any padding;        Re-segmentation of PDUs that need to be retransmitted: if a retransmitted PDU does not fit entirely into the new TB used for retransmission then the RLC PDU is re-segmented;        The number of re-segmentation is not limited;        Concatenation of SDUs for the same radio bearer;        In-sequence delivery of upper layer PDUs except at Handover (HO) in the uplink;        Duplicate Detection;        
FIG. 3 shows the RLC PDU structure where the PDU sequence number carried by the RLC header 310 is independent of the SDU sequence number (i.e. PDCP sequence number). The division lines in SDUn 320 and SDUn+3 330 indicate occurrence of segmentation. Because segmentation only occurs when needed and concatenation is done in sequence, the content of an RLC PDU can generally be described by the following relations:                {0; 1} last segment of SDUi+[0; n] complete SDUs+{0; 1} first segment of SDUi+n+1; or        1 segment of SDUi.        
FIG. 3 and FIG. 4 describe some of the RLC functions. The RLC layer receives RLC SDUs from the layer above it (i.e. PDCP); based on the size of the Transport Block (TB) to be sent in a Transmission Time Interval (TTI), the RLC either segments the SDU, leaves the SDU as is, or creates a concatenation of SDUs and segments 410, in a manner that maximizes the utilization of the TB. This results in an initial RLC PDU that gets transmitted. The initial RLC PDU 420 (or its constituent information) will also be kept in a retransmission (ReTx) buffer 430 until it is successfully acknowledged. In case of errors or lack of reception, the unacknowledged PDU may be retransmitted by ARQ either as is, or may undergo PDU re-segmentation 440 where the PDU is further segmented into sub-PDU's for retransmission 450. Hence the RLC retransmission unit is the RLC PDU or an RLC Sub-PDU depending on whether re-segmentation is performed or not.
In the RLC PDU re-segmentation scheme, there are two levels of identifications employed by the RLC:                1) RLC PDU SN to identify the RLC PDU        2) RLC Sub-PDU identifier, which could take the form of either:                    a. SN, e.g. Sub-PDU SN, or:            b. Offset and Length, e.g. in Bytes, relative to the PDU.                        
In the RLC SDU re-segmentation scheme, there are two levels of identifications employed by the RLC:                1) RLC SDU SN to identify the RLC SDU        2) RLC Sub-SDU identifier, which could take the form of either:                    a. SN, e.g. Segment SN or Sub-SDU SN, or:            b. Offset and Length, e.g. in Bytes, relative to the SDU.                        
PDU re-segmentation is described, but SDU re-segmentation is also possible. FIG. 5 illustrates an example implementation that performs SDU re-segmentation.
What is needed are different alternatives for efficient and/or low complexity user plane architectures designed to support ciphering operation in an efficient and/or low complexity manner; where the prior art does not provide alternatives of MAC and RLC ciphering.
In UMTS systems, there was only one level of RLC segmentation, and there was no RLC re-segmentation. LTE RLC re-segmentation introduces problems in regards to how the ciphering SN (C-COUNT) is to be constructed efficiently. This is further complicated by being dependent on the various numbering schemes that may be employed by RLC for other functions such as ARQ and reordering.